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Your Rossi 21 engine is the older 5-port version.
I recommend the non-muffled MACS tuned pipe # 1120 for nice performance. Distance between glow plug and center of the widest part of the pipe is 8.5" for the first test runs - to be shortened down to 8" in 5 mm increments. Rossi R6 plugs. Start with 10 % nitro fuel. 6.5*6.5 APC pylon prop when using a slim fuse. Expect static rpms of 26K.
Tip: Replace the R/C carb by a venturi carb. Good luck!